The present invention relates to a vehicle floor frame structure that is provided in a lower portion of a vehicle body, and which includes a floor panel, side members, and the like.
In a common vehicle floor frame structure, a floor panel is provided in an entire lower portion of a vehicle body, and side members extending in a vehicle body front-and-rear direction are provided on both left and right sides in a vehicle body width direction of the floor panel. Of these, the side members are frame members extending over almost the entire length of the vehicle body in the vehicle body front-and-rear direction, and are important in securing the rigidity of the vehicle body. Moreover, the side members play a principal role in transmitting a load input from the front side of the vehicle body to members at the rear side of the vehicle body.
In this respect, in order to efficiently transmit a load from the front side of the vehicle body to the members at the rear side of the vehicle body, it is important to employ a structure in which the side members positioned on both the left and right sides extend with a distance therebetween maintained constant without being bent locally. This structure enables the floor frame structure to have a necessary rigidity, and light-weight members to be employed for the side members and the like.
However, in a basic frame structure of a small vehicle with a compact design, the distance between the side members in the vehicle body width direction is also small because of a small vehicle width. Meanwhile, there is a structure in which rigid members are disposed to concentrate on an inner side in the vehicle body width direction to effectively receive a load input from the front side of the vehicle body. When such a structure is employed, the distance between the side members in the vehicle body width direction tends to be small.
Nevertheless, in the vicinity of a rear-floor, rear side members in the rear side of the vehicle body are required to be positioned on outer sides in the vehicle body width direction for the purpose of avoiding interference with members such as a fuel tank disposed in the rear side of the vehicle body, or attaching rear tire suspensions. For this reason, a conventional floor frame structure employs side members each having a vehicle body rear side shaped to curve outward further from the center in the vehicle width direction or is provided with additional members extending in the vehicle body front-and-rear direction. With such a floor frame structure, a load input from the front side of the vehicle body is transmitted to the rear side members at the rear side of the vehicle body, so that the load is absorbed by the vehicle body as a whole.
Meanwhile, the basic frame structure of a small vehicle with a compact design is required to avoid increase in weight due to additional members. In this respect, in a type of conventional frame structure, each front side member is curved outward to be continuous to a side member of a main floor, and the side member of the main floor is maintained at the outer side position in a vehicle body and is continuous to a rear side member at the rear of the vehicle body (see, for example, JP 2009-12677 A).
In another type of conventional frame structure, front side members are continuous to side members of a main floor, while maintaining the distance in the vehicle body width direction almost the same, and the side members of the main floor are curved outward in the vehicle body to be continuous to rear side members at the rear of the vehicle body (see, for example, JP 2005-219521 A).
Moreover, in still another type of conventional frame structure, each side member is divided into a floor side member for a main floor and a rear side member for a rear floor, a rear portion of the floor side member is connected to the rear floor without being curved, and the rear side member is provided at a position shifted from the floor side member in the vehicle body width direction (see, for example, JP 2008-13078 A).
In the conventional floor frame structure shown in JP 2009-12677 A, however, a curved portion is formed in the front side member in the front side of the vehicle body which is required to absorb a large load. Hence, it is also necessary to reinforce the curved portion, and to design the floor frame structure so that the side member may not bend in the curved portion when being deformed in any pattern.
Meanwhile, especially in the case of the basic frame structure of a small vehicle with a compact design, the conventional floor frame structure shown in JP 2005-219521 A sometimes lacks necessary frame rigidity, because the length in the vehicle body front-and-rear direction of the curved portions of the side members is so short that the side members are sharply curved outward. Moreover, there is a problem that press molding of such side members is difficult, which leads to decrease in productivity.
Moreover, in the conventional floor frame structure shown in JP 2008-13078 A, a load inputted from the front side of the vehicle body is transmitted from the front side members to the floor side members, but is not transmitted to the rear side members in the rear side of the vehicle body. In other words, the load is received by the front portion of the rear floor, and stops at the front portion. As a result, the load tends to concentrate at the front portion of the rear floor, which raises a concern about buckling deformation of the rear floor. For this reason, additional reinforcing means needs to be provided at the position of the rear floor.